Field of the Invention
The invention relates to a process for producing a catalyst body, including providing a kneadable and/or shapable compound including titanium dioxide, tungsten trioxide, vanadium pentoxide, aluminum oxide, and/or silicon oxide from starting materials. The kneadable and/or shapable compound is processed into a shaped body by extrusion or by coating of a support body, and the shaped body is then calcined to form an active compound from the compound.
A production process is disclosed, for example, from U.S. Pat. No. 4,085,193 to Nakajima et al. A catalyst body that is produced according to Nakajima et al. is distinguished by a high activity in terms of breaking down nitrogen oxide in a gaseous medium using the selective catalytic reduction process (SCR process). The nitrogen oxides contained in the gaseous medium, also in the presence of oxygen, are broken down by a reducing agent to form water and elemental nitrogen.
German Utility Model DE 296 15 192 U1 also discloses a production process. To apply further active components, the document proposes deep impregnation of the calcined shaped body and a final thermal after-treatment.
Furthermore, German Published, Non-Prosecuted Patent Application DE 35 20 024 A1 discloses a production process for a SCR catalyst that does not contain any titanium dioxide. During production, calcination is carried out in two stages.
In such production processes, the kneadable and/or shapable compound is produced either as an aqueous slurry from the above-mentioned materials in powder form or from the above-mentioned materials in the form of their precursor compounds, such as, for example, oxo acids, by removal of water. Such production is extensively described in the above-mentioned U.S. Patent to Nakajima et al. During production, further auxiliaries and/or fillers may be added to the above-mentioned materials. On one hand, these additions facilitate processing to form the kneadable and/or shapable compound and, on the other hand, have a beneficial effect on the stability of the finished active compound. Examples of such auxiliaries include ceramic fillers or certain organic or organosilicon fibers. In particular, organic fibers are also used as pore-forming agents because the included organic fibers burn during the final calcination process.
A catalyst body based on titanium dioxide, tungsten trioxide, and/or vanadium pentoxide, aluminum oxide, and silicon oxide that has been produced using the prior art process is usually referred to as a deNOx catalyst. Such a catalyst body is used in the form of a shaped body through which medium can flow, for example, in the form of a honeycomb or a plate or in the form of pellets or a bed of bulk material, to break down nitrogen oxides in the exhaust gas from a combustion installation. Because the presence of oxygen also does not represent any problems with regard to the reduction of nitrogen oxides, such a catalyst body is also used to reduce nitrogen oxides in the exhaust gas from a diesel engine. Such a catalyst body can also be used to remove nitrogen oxides from the exhaust gas of a fossil-fired power plant, a refuse incineration plant, or a gas turbine. However, use as a diesel catalyst is becoming increasingly important.
Significantly, temperatures of up to at most 650xc2x0 C. occur in the exhaust gas from a diesel engine. The catalyst produced in the prior art, however, is substantially only suitable for use at a temperature of up to or below 450xc2x0 C. if it is to be used to reduce the levels of nitrogen oxides using the SCR process. Above this temperature, increasing deactivation of the active compound takes place. There is an irreversible reduction in its activity. Thereafter, sufficiently high catalytic activity to break down nitrogen oxides is no longer ensured even at lower temperatures. Particularly in the case of a diesel engine that is used to drive a vehicle and is, therefore, exposed to high load changes, the active compound of the catalyst body becomes deactivated relatively quickly.
It is accordingly an object of the invention to provide a process for producing a catalyst body based on titanium dioxide that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type where the catalyst body is able to withstand temperature loads of from 400xc2x0 C. to 650xc2x0 C.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a process for producing a catalyst body, including the steps of producing a compound from starting materials including tungsten trioxide, vanadium pentoxide, titanium dioxide, aluminum oxide, and silicon oxide, processing the compound into a shaped body by one of extrusion and coating of a support body, forming an active compound from the compound by calcinating the shaped body at a calcination temperature, and artificially aging the calcined shaped body by a final heat treatment at a temperature, higher than the calcination temperature, of from 620 to 720xc2x0 C. for a total duration of from 20 to 80 hours. The compound is kneadable and/or shapable.
The invention achieves its objective by the fact that, compared to a production process according to the prior art, the calcined shaped body is subjected to artificial aging by a final heat treatment at a temperature of from 620xc2x0 C. to 720xc2x0 C., a temperature that is higher than the calcination temperature, for a total duration of from 20 to 80 hours.
The term calcination is understood as meaning a temperature treatment in which the starting materials combine to form a microstructure of crystallites, in which the volatile components, for example, water of crystallization, are removed, and in which a mechanically stable solid body that undergoes scarcely any change in volume during further heat treatment is formed. Heating the shaped body and holding it at the required calcination temperature, for example 400 to 700xc2x0 C., for a certain holding time, for example 1 to 20 hours, produces the calcination.
Surprisingly, it has been found that a heat treatment has a positive effect on the calcined shaped body, such that, as a result, the catalyst body acquires a higher resistance to deactivation at high temperatures. It is not currently known what chemical changes to the active compound cause the result. It is possible that the controlled heat treatment leads to a higher degree of linking of the individual components of the active compound of the catalyst body, thus preventing deactivation. It is possible that the treatment also prevents recrystallization of the titanium dioxide from the anatase modification to the rutile modification, with a loss of specific surface area.
In an advantageous configuration of the invention, the final heat treatment is carried out at a temperature of from 620xc2x0 C. to 720xc2x0 C. The temperature is significantly higher than the temperature at which the catalyst body is customarily used in SCR applications. The temperature range has proven particularly advantageous with regard to the service life of the catalyst body. A temperature range of from 660xc2x0 C. to 700xc2x0 C. has proven particularly advantageous.
In accordance with another mode of the invention, it is advantageous if the overall duration of the final heat treatment is from 20 to 80 hours. The maximum stability of the catalyst body is generally reached after about 35 hours. The heat treatment can be carried out either continuously or discontinuously with interruptions.
In accordance with a further mode of the invention, it is expedient for the production process if the shaped body is dried at a temperature of between 20xc2x0 C. and 100xc2x0 C. prior to the calcination.
In accordance with an added mode of the invention, it is advantageous if the shaped body is calcined at a temperature from 400xc2x0 C. to less than 700xc2x0 C.
In accordance with a concomitant mode of the invention, the catalyst body includes 65 to 95% by weight of titanium dioxide, 2 to 30% by weight of tungsten trioxide, 0 to 2% by weight of vanadium pentoxide, 0.1 to 10% by weight of aluminum oxide, and 0.1 to 10% by weight of silicon dioxide. Binders and fillers may be added in a conventional manner. It is particularly advantageous with a view to achieving resistance to deactivation at high temperatures if the vanadium pentoxide content is lower than 1.5% by weight.
The catalyst body should be produced such that it has a specific surface area, measured as the BET surface area, of from 30 to 150 m2/g. The pore volume, measured using the mercury penetration process, should be between 100 and 1000 mm3/g, with a monomodal or polymodal pore radius distribution.
The catalyst body produced can be used with a very wide range of geometries. For example, it can be used in honeycomb or plate form, in the form of pellets, or in the form of a bulk bed. The catalyst body is particularly suitable for use as a deNOx catalyst in the exhaust gas from a diesel engine or in the exhaust gas from a gas turbine.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for producing a catalyst body based on titanium dioxide, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
The FIGURE is a graph indicating the way in which the temperature stability of the catalyst body is dependent on the vanadium pentoxide content and the duration of the final heat treatment according to the invention.